1. Field of the Invention
This invention relates to a photoelectrical switching circuit for detecting light rays emitted by a light emitting circuit by a light detecting circuit and detecting the presence or absence of an object in the optical path, and more particularly to a photoelectrical switching circuit for preventing malfunctioning in a detecting operation, which is formed in a simple circuit configuration and suitable for high integration and size reduction of the circuit.
2. Description of the Related Art
Conventional photoelectrical switches are constructed such that light rays, which, for example, have been pulse-modulated, are emitted from a light emitting circuit, that the light rays being reflected or blocked by an object is detected by the presence or absence of the same pulse-modulated light rays incident on a light detecting circuit, and that a detection signal is output from an output circuit.
Various contrivances have been made to prevent malfunctioning in the photoelectrical switching circuits. There has recently been a requirement for improvements in the photoelectrical circuit in order to meet the need to supply mass-produced products which are small in size and less expensive.
In conventional photoelectrical switches, the supply voltage just after the power supply unit is turned on is unstable or lower than a required voltage level, which causes an unstable operation of the amplifier, etc. of the detection circuit, and sometimes causes an erroneous detection signal to be output. As a counter-measure, the power supply unit has been arranged not to supply power until the supply voltage settles to a required high voltage. The generally adopted method is to connect a capacitor through a resistor to the output side of the power unit and not to supply power until the voltage across the capacitor exceeds a certain reference voltage when supplying the power. When there is optical or electrical noise which continues for a short time, the light detector malfunctions because the light detector takes the noise as a signal produced by blocking or reflecting by the object of the light rays emitted from the light emitter. To prevent this malfunctioning, it is necessary to make an arrangement in which a detection signal is not issued until the output of the light detector has continued for a predetermined period of time. The method which has been practiced is to input a pulse signal from the light detector to an integrating circuit and, compare the output of the integrating circuit with a specified level, and then output a detecting signal as ON/OFF switching signal.
For a measure for preventing malfunctioning, it is necessary to provide the power supply circuit with a capacitor of a relatively large capacity and provide the integrating circuit with a reactance element such as a large capacitor or coil. For this reason, it is difficult to produce the power supply circuit in a one-chip integrated circuit (IC). Since terminals for connecting a capacitor or a coil need to be provided on the IC, the IC has to be large in size, and costs of material and assembly tend to be large. In addition, the time constant of the integrating circuit varies with the individual reactance elements having different properties, thus giving rise to errors in the operation, which has been a problem.
In view of the above situation, Oi et al. disclose in JP-A-242417/86 a photoelectrical switch for achieving the power supply circuit integrated in one-chip IC by eliminating a capacitor from the power supply circuit. The photoelectrical switch furnishes its circuit with a counter and a gate circuit. The counter counts clock pulses output from a clock: generator circuit, and issues an operation signal to activate its output circuit, when the count reaches a predetermined value. And the gate circuit blocks the input cf a clock pulse to the counter after the operation signal is output from the counter. By this arrangement, an unstable period of the supply voltage just after turning on the photoelectrical switching circuit is eliminated securely, and once the power supply has become stable, the counter stops, then the photoelectrical switch operates normally.
In JP-A-82066/77 (JP-B-28049/81), Fukuyama et al. disclose a circuit to output a detection signal when continuation of detecting pulse input is confirmed for a certain period which is provided with a serial input-parallel output type shift register, AND gates and a flip-flop, without a reactance element for preventing an erroneous detection in the light detecting circuit. The shift register receives pulse signals from the light detecting circuit serially with fixed time intervals, the AND gates decide state of the outputs from each stage of the shift register, and the flip-flop is set when all outputs are "1", and reset when all outputs are "0". And the output of the flip-flop is used as a detection signal of a switching output. By this arrangement, a presence-absence decision is not made before the same signals are input continuously up to the same number of the stages of the shift register, thereby a correct detection may be surely obtained.
By the above disclosures, it has become possible to eliminate a reactance element, such as a large capacitor or coil, make a power supply circuit on a one-chip IC, and set an operation time accurately.
Meanwhile, in ordinary photoelectrical switches, it is necessary to provide a circuit for generating pulses to drive the light emitting circuit to produce modulated light rays and to drive a gate to confirm that the light rays received by the light detecting circuit are the same light rays emitted by the light emitting circuit of the same photoelectrical switch. For this purpose, a relatively small and less expensive high-frequency pulse generator circuit is used. The clock pulses generated by this high-frequency pulse generator are subjected to frequency division by the frequency divider circuit to obtain desired frequency pulses. This frequency divider circuit has a function similar to that of the above-mentioned counter developed by Oi et al., which is to be attached to the power supply circuit. As described above, according to the idea by Oi et al., there exist two similar circuits within the whole circuit, a fact which causes the problems of the large circuit size, the excessive material cost, and the superfluous assembly processes.
According to Fukuyama et al., it is necessary to provide the same number of AND gates as the number of stages of the shift register for accepting their parallel outputs. Therefore, when the delay time is varied, it is required to make an extensive change of the circuit configuration to increase or decrease input lines to the AND gates, as well as to increase or decrease the number of stages of the shift register, so that a change of delay time cannot be done easily. A conventional photoelectrical switch, which is arranged to detect the output of the light detecting circuit through a gate driven in same phase of driving pulses for driving its own light emitting circuit, sufficiently prevents malfunctioning due to external leakage light, when it is used separately. However, when a plurality of photoelectrical switches of the same type are installed in a row, an erroneous detection often occurs which is ascribable to the incident light rays having a similar phase coming from the adjacent photoelectrical switches. To prevent this erroneous detection, there have been some photoelectrical switches which have been designed to use light emission pulse signals out of phase with one another. Nevertheless, with those photoelectrical switches, an available variety of periods of the light emission pulse signal that can be selected is limited, so that in the light detecting circuit, the read timing often coincides with the light emission timing of other photoelectrical switch, making it difficult to eliminate the effects of light from the adjacent photoelectrical switches. To avoid this problem, in some switches, the light emission angle of the light emitter and the light detecting range of the light detector have been narrowed. In this case, the demand on the mounting accuracy of the parts for correct alignment of the optical axis is very stringent, and the operational stability is vulnerable to the positional displacement sometimes occurs caused by a shock applied during operation. In addition, there has been a system in which the light emission timing is shifted in a group of photoelectrical switches. To put this system into smooth operation, the system configuration have to be complicated because it is necessary to provide control means to co-ordinate the actions of the whole system. Moreover, wiring works in site are required to connect the control means with all individual photoelectrical switches in the place where they are installed.
So Fukuyama et al. disclose in JP-A-14718/82 (JP-B-51043/85) a photoelectrical switch for preventing malfunctioning of the light detecting circuit by having the light emission timing of its own differ from the light emission timing of another photoelectrical switch by making long or short the interval of pulses of the reference clock pulse generator when noise from another photoelectrical switch is detected.
Even by the disclosure by Fukuyama et al., owing to their limited numbers of available frequencies, when photoelectrical switches of the same type are installed side by side, when the switches are turned on nearly at same timing, and when just after the switch detects the emitted light from another switch, there is a tendency for the phases of the emitted light pulses of the neighboring photoelectrical switches to come closer, so that malfunctioning is likely to occur.